I don't think you can flash K800 to W890... There are MANY differences that make the firmware incompatible with K800. First it is different platform... We re not 100% sure about that if the W890 is DB3150 like the K850, but anyway the second reason is the hardware difference. The w890 hasn't got joystick... the keys are a bit different... Simply, I don't think is is possible...
P.S. For the K770 firmware in K800, if someone flashed it successfuly and if the phone is working as above was mentioned that XS++ fails on customizing the phone, you can try opening it with K800 script in far manager... and then customize the phone...